As devices that collect a sound wave and convert the sound wave to an electric signal, a dynamic microphone or a capacitor microphone has been widely used in the audible band, and a piezoelectric sensor has been widely used in the ultrasonic region. In these devices, by utilizing the fact that a sound wave is derived from minute vibrations of air, the sound wave is made incident on a diaphragm so that minute vibrations generated on the diaphragm by the sound are converted to an electric signal conductively, electrostatically, or piezoelectrically.
On the other hand, an optical system, such as a laser Doppler vibrometer (hereinafter, referred to simply as ‘LDV’) that measures minute, high-speed vibrations by utilizing light typically represented by laser light, has been widely used, and an attempt has been made to collect sound waves by utilizing such a device.
In a sound pressure converting device as described in Patent Document 1, an optical microphone has been proposed in which a diaphragm used in a normal microphone and optical measurements by the use of an optical triangle method are applied.
Moreover, Patent Document 2 has disclosed a laser Doppler microphone in which, by directly propagating laser light in a sound field, a change in refractive index caused in the air by a sound wave is directly caught by the LDV so that a sound pressure is measured.
Referring to FIG. 11, the following description will discuss the structure and operations of the laser Doppler microphone in Patent Document 2. In FIG. 11, reference numeral 121 represents an LDV, 122 and 123 represent a pair of reflective mirrors, 124 represents a cubic mirror, 125 represents a laser optical path, 126 represents a sound field, and 127 represents an operation unit.
In the structure as shown in FIG. 11, the paired reflective mirrors 122 and 123 are disposed in parallel with each other, and the LDV 121 and the cubic mirror 124 are arranged on the upper and lower ends of one of the reflective mirrors 123. Laser light is projected from the LDV 121 with an appropriate angle toward the reflective mirror 122. The laser light thus projected is propagated along the laser optical path 125, while being reflected by the reflective mirrors 122 and 123 a plurality of times, to reach the cubic mirror 124 arranged on the terminal of the reflective mirror 123. The laser light made incident on the cubic mirror 124 is emitted from the cubic mirror 124 toward the incident direction of the laser beam onto the cubic mirror 124 after having been reflected inside the cubic mirror a plurality of times, and is again propagated through the laser optical path 125 in the reversed direction, while being reflected by the paired reflective mirrors 122 and 123 a plurality of time, to reach the LDV 121. The laser light thus reached the LDV 121 is subjected to optical and electrical treatments inside the LDV 121, and its vibration velocity component is converted by the operation unit 127.
In the structure of FIG. 11, since no vibrating portions are present, normally, the vibration velocity component is zero. In a case where a sound wave is present in a space formed by the reflective mirrors 122 and 123, decreased and increased density portions are generated in the density of air by the sound wave. These changes in the density cause changes in the refractive index in the air and subsequent changes in the propagation velocity of the laser light, with the result that the velocity component corresponding to the sound wave is measured as if vibrations occurred in the reflective mirrors 122 and 123 or in the cubic mirror 124.
Moreover, in Patent Document 3, the present inventors have disclosed an invention relating to an ultrasonic wave transmitter/receiver for a gas that can transmit and receive ultrasonic waves in a wide band with high sensitivity by utilizing the refraction of the ultrasonic wave in the gas. Furthermore, Non-Patent Document 1 has reported transmitting and receiving characteristics in an ultra high frequency region of 500 kHz.
FIG. 12 is a schematic view that shows the ultrasonic wave transmitter/receiver of the inventions disclosed in Patent Document 3 and Patent Document 4.
As shown in FIG. 12, the ultrasonic wave transmitter/receiver 101 of the invention of Patent Document 3 is provided with at least an ultrasonic vibrator 102 and a propagation medium portion 103 that is placed on the front face of the ultrasonic vibrator 102 so that the gap between an environmental fluid 104 and the ultrasonic vibrator 102 is filled therewith. Reference numeral 105 indicates an advancing direction of an ultrasonic wave. The ultrasonic wave transmitter/receiver in this aspect is particularly referred to as a refraction propagation-type ultrasonic wave transmitter/receiver (or diagonal propagation-type ultrasonic wave transmitter/receiver main body).
In this case, the interface between the ultrasonic vibrator 102 and the propagation medium portion 103 is defined as a first surface region 11, and the interface between the propagation medium portion 103 and the environmental fluid 104 is defined as a second surface region 12.
The refraction propagation-type ultrasonic transmitter/receiver of Patent Document 3 is designed so that an ultrasonic wave can be transmitted and received with high sensitivity by receiving an ultrasonic wave with high efficiency to the propagation medium portion of a propagation medium from a medium that is extremely small in its acoustic impedance such as air.
Normally, on the interface between media such as a gas and a solid substance that are greatly different in their acoustic impedances, almost all the ultrasonic waves are reflected, failing to transmit or receive the waves with high sensitivity. In an attempt to realize transmission of an ultrasonic wave in such a gas with high efficiency, the refraction propagation-type ultrasonic wave transmitter/receiver 101 forms an ultrasonic wave transmitter/receiver 101 by utilizing a propagation medium portion 103 made of a special material. The propagation medium portion 103 needs to have characteristics having a sound velocity that is slower than that of the environmental fluid 104 and a density that is greater than that of the environmental fluid 104, and Patent Document 3 uses a dried gel material made of a silica skeleton as such a material. The dried silica gel is a material that is allowed to have various sound velocities and densities by adjusting its manufacturing processes, and, for example, this material can take values that satisfy conditions of the propagation medium portion 103 that can transmit an ultrasonic wave with high efficiency, such as, for example, a density of 200 kg/m3 and a sound velocity of 150 m/s.
By using this material as the propagation medium portion 103, with an angle θ1, made by the normal to the second surface region 112 and the ultrasonic wave propagation direction inside the propagation medium portion 103, and an angle θ2 made by the ultrasonic wave propagation direction inside the environmental fluid 104 being respectively selected appropriately, as shown in FIG. 12, the reflection of the ultrasonic wave inside the second surface region 112 is made substantially zero so that it becomes possible to achieve an ultrasonic wave transmitter/receiver with high wave transmitting and receiving sensitivity. Moreover, since the frequency of a sound wave has no relationship with the transmitting efficiency in the second surface region 112, it is possible to achieve a wide band characteristic, on principle, and also to measure various frequencies with high efficiency.
More specifically, upon transmitting an ultrasonic wave, an electric signal is given to the ultrasonic vibrator 102 from a driving circuit not shown to generate an ultrasonic wave. In this case, X, Y, and Z directions are defined as shown in FIG. 12. The ultrasonic wave generated in the ultrasonic vibrator 102 is propagated through the propagation medium portion 103 from the first surface region 111 to the second surface region 112 in the positive direction of the Y-axis. The ultrasonic wave thus reached the second surface region 112 is changed in its propagation direction in accordance with the law of refraction, and then propagated into the fluid 104 toward the direction of the ultrasonic wave propagating path 105 (in this case, direction opposite to the arrow).
Upon receiving an ultrasonic wave, in a manner reversed to the case for transmitting the ultrasonic wave, when the ultrasonic wave propagated through the fluid 104 in the ambient space has reached the second surface region 112, it is refracted and allowed to transmit through the propagation medium portion 103, and is propagated through the inside of the propagation medium portion 103 in the negative direction of the Y-axis to reach the ultrasonic vibrator 102. The ultrasonic wave thus reached the ultrasonic vibrator 102 deforms the ultrasonic vibrator 102 so that an electric potential difference is generated between electrodes, and detected by a wave-receiving circuit, not shown.
In the diagonal propagation-type ultrasonic wave transmitter/receiver main body 101, even in a case where the fluid 104 is a medium that has an extremely small acoustic impedance (sound velocity of a material×density of the material), such as air, an ultrasonic wave can be made incident on the propagation medium portion 103 from the fluid 104 with high efficiency, or can be emitted from the propagation medium portion 103 to the fluid 104 with high efficiency.
In the diagonal propagation-type ultrasonic wave transmitter/receiver main body 101, in an attempt to enhance the transmitting efficiency of ultrasonic waves, a sound velocity C1 in the propagation medium portion 103 for the ultrasonic wave, a sound velocity C2 in the fluid 104 for the ultrasonic wave, a density ρ1 of the propagation medium portion 103, and a density ρ2 of the fluid 104 are set so as to satisfy the following expression (2):[Expression 2](ρ2/ρ1)<(C2/C1)<1  (2)
Moreover, by using C1, C2, ρ1, and ρ2, θ1 is set so as to satisfy the following expression (3):[Expression 3](tan θ1)2=[(ρ2/ρ1)2−(C1/C2)2]/[(C1/C2)2−1]  (3)
Moreover, there is a relationship indicated by the following expression (4) between θ1 and θ2.[Expression 4]sin θ1/C1=sin θ2/C2  (4)
As shown in Patent Document 4, when the above-mentioned expressions (2), (3), and (4) are satisfied, the transmitting efficiency of the ultrasonic wave in the second surface region 112 becomes substantially 1. Consequently, it becomes possible to provide a diagonal propagation-type ultrasonic sensor serving as a diagonal propagation-type ultrasonic wave transmitter/receiver main body 101 that can transmit and receive an ultrasonic wave with high efficiency.    Patent Document 1: JP-A No. 2004-12421    Patent Document 2: JP-A No. 2004-279259    Patent Document 3: WO No. 2004/098234    Patent Document 4: Specification of US. Patent Application Laid-Open No. 2005/0139013    Non-Patent Document 1: “Acoustic Properties of Nanofoam Material and its Applied Ultrasonic Sensors” (HASHIMOTO Masahiko, NAGAHARA Hidetomo, SUGINOUCHI Takehiko), Technical Research Report of The Institute of Electronics, Information and Communication Engineers, issued by Incorporated Company, The Institute of Electronics, Information and Communication Engineers, Vol. 105, No. 619, US2005-127 (P. 29-34)